Several million years ago, long before the first humans walked the planet, dinosaurs were the most dominant species on Earth.
They ruled the terrestrial ecosystem for more than 170 million years.
Scientists have been able to trace the evolution of this unique group of reptiles throughout geologic history and reconstruct their family trees as they evolved over the course of the Mesozoic Era.
Sixty-six million years down the line, all records of the dinosaurs vanished.
It was sudden and unexpected, a phenomenon that has continued to puzzle scientists all over the world.
The extinction of the dinosaurs wasn’t an isolated event.
Throughout geologic history, there have been five big mass extinction events and several other small ones.
Every few million years, some of the Earth’s fauna and flora disappear.
The event is typically triggered by various factors, but it’s a necessary part of the Earth’s natural cycle.
However, while the extinction of dinosaurs is somewhat normal, scientists are still puzzled by how abrupt it was.
Even more puzzling is the fact that we can’t say for sure why it happened.
As with every scientific question, different theories have been put forward.
The most popular one is that an asteroid impact 66 million years ago caused the catastrophe that ended all non-avian dinosaurs along with 75% of life on Earth.
Other theories, such as climate change and volcanic eruptions, have been put forward—and these are the sane ones.
Some weirder theories, like alien attacks or destructive caterpillars, used to be popular in the past.
Regardless of the cause, the extinction of the dinosaurs remains one of the biggest scientific questions.
In this article, we’ll explore this question, presenting some of the most popular theories and alternative ones that seek to answer the question of what killed the dinosaurs.
The Reign of Dinosaurs
Dinosaurs are arguably the most successful animal group to have ever lived.
They were the most dominant group on the planet for about 170 million years.
Their existence spanned the entire Mesozoic Era, which has been nicknamed the age of dinosaurs.
The earliest of these beasts evolved during the Late Triassic Period.
The ancestors of the dinosaurs were odd and unassuming creatures.
They were mostly small bipedal animals such as Eodromaeus, Eoraptor, Nyasasaurus, and Herrerasaurus.
These small creatures, which were just a few feet long, soon evolved into some of the largest animals to have ever walked the planet.
The rise of the dinosaurs during the Triassic Period followed one of Earth’s most devastating extinction events, the Permian-Triassic extinction event.
This event, also known as the great dying, is regarded as the worst extinction event in Earth’s history.
From the ashes of this event, new creatures, including the ancestors of present-day mammals and various reptile groups, emerged to fill the vacant niche now left behind.
They all lived on the supercontinent Pangea which was just starting to break up at the time.
The Jurassic Period saw the rise of dinosaurs to prominence.
Boosted by a small extinction event at the end of the Triassic that killed off many of their competition, the dinosaurs diversified rapidly into various sizes and filled different ecological niches.
The continents were still connected, so they were able to spread quickly to cover the landmass of all the continents.
By the middle of the Jurassic Period, dinosaurs became the undisputed top dogs in the ecosystem.
The first true giants of this period emerged in the form of long-necked herbivorous sauropods and massive theropods.
They were a big upgrade compared to their man-sized ancestors of the Triassic Period.
But they weren’t the only successful animal group during this period.
Dinosaurs attained their maximum diversity during the Cretaceous Period.
During the Cretaceous Period, the continents began to take on a form similar to their present-day positions.
But unknown to Earth’s most successful creatures, danger was on the horizon, and things were about to take an unfortunate turn.
The next series of events led to the decline and eventual disappearance of the non-avian dinosaurs.
The Cretaceous-Paleogene (K-Pg) Extinction Event
The extinction event that ended the successful run of the dinosaurs is known as the Cretaceous-Paleogene (K-Pg) extinction.
It is sometimes referred to as the K-T (Cretaceous-Tertiary) extinction event.
This event marks the end of the Mesozoic Era and the transition to the Cenozoic Era about 66 million years ago.
The non-avian dinosaurs were the most notable victims of this extinction event but were not the only ones affected.
Basically, all four-legged terrestrial animals that weighed more than 25 kilograms (55 pounds) died in this event (with the exception of turtles and crocodilians).
In the oceans, mosasaurs and plesiosaurs didn’t survive this devastating extinction event.
Ultimately, up to 75% of life on the planet was gone.
In the geologic record, the Cretaceous-Paleogene (K-Pg) Extinction Event is marked by the abrupt absence of the dinosaurs and several other plant and animal groups in subsequent fossil records.
Scientists have also noted a thin layer of sediments marking the boundary between the Mesozoic and Cenozoic eras.
This thin layer of sediment is found in both marine and terrestrial rocks all over the world.
The sediment shows high levels of the metallic element “iridium,” which will prove valuable in determining the cause of the K-T extinction, as you will see later in this article.
There are two schools of thought regarding the cause of the end-Cretaceous extinction event.
The intrinsic gradualists believe the event occurred gradually over a period of several million years and was caused by earthly factors.
The extrinsic catastrophists, on the other hand, think the event occurred suddenly and was caused by extraterrestrial factors.
The Impact Theory
The impact theory is the most popular theory to explain the cause of the dinosaur extinction, and it falls in the extrinsic catastrophe category.
This theory has many variations, but the main one was first proposed in 1980 by a team led by Nobel Prize-winning physicist Luis Alvarez and his geologist son Walter Alvarez.
They proposed that the extinction of the dinosaurs was caused by a massive extraterrestrial object that crashed into the earth 66 million years ago.
The massive rock, which was as big as Mt. Everest, caused a blast that killed off all animals in the immediate vicinity.
The collision released energy as high as 100 teratonnes of trinitrotoluene (TNT), which is more than a billion times the energy of the atomic bombs that destroyed Hiroshima and Nagasaki.
It also threw up enough dust to have caused significant climate change in the years that followed the initial impact.
One of the key pieces of evidence that pointed Alvarez and his team in this direction was the discovery of a globally distributed layer of sediment enriched in iridium.
Iridium is a rare metal in Earth’s crust but common in asteroids and comets.
This layer, found at the K-Pg boundary, provided strong support for an extraterrestrial impact event.
But an extraterrestrial impact of the magnitude proposed by Alvarez is expected to leave a notable impact crater.
The crater would be 65 million years old and was expected to measure at least 100 kilometers (about 65 miles) in diameter.
For many years after this theory was first proposed, no crater big enough and of the right age to fit this theory was found.
In 1990, about a decade after the theory was initially proposed, a crater fitting that description was discovered on the Yucatan Peninsula of Mexico.
Called the Chicxulub crater after the town of Chicxulub in Mexico, the oval impact crater was measured to be about 180 kilometers (110 miles) in diameter and about the same size as the crater suggested by the Alvarez team.
The Chicxulub crater was of the right age too.
In addition to the iridium layer and the discovery of the crater itself, scientists have also found other evidence to support the impact theory.
This includes the presence of shocked quartz (rearranged crystals of quartz grains caused by intense pressure and temperature due to an impact event) in some of the rocks of the K-Pg boundary.
The Alvarez team proposed that the impact would have released massive amounts of energy, creating fires that released soot and aerosols into the atmosphere.
The debris, as well as the vaporized sulfur and hydrocarbons produced when the asteroid hit the layer of carbonate rocks, would have caused a “nuclear winter” effect, blocking sunlight for up to a year and forming acid rain.
This led to a significant drop in global temperatures for up to three years.
This environmental disruption that followed the initial impact could have led to the collapse of ecosystems and the mass extinction of dinosaurs and other animals within a short period.
Scientists have found soot layers in sediments dated to the Late Cretaceous Period, which provides evidence of massive hurricanes and firestorms that would have ravaged the planet in the aftermath of the asteroid impact.
Volcanic Activity and Deccan Traps
Scientists that believe in the intrinsic gradualist theory often hinge their theory on drastic environmental changes that occurred as a result of volcanic activity.
This isn’t the first time volcanic activities will be blamed for an extinction event.
In fact, the great dying is believed to have been caused by catastrophic volcanic eruptions.
For the K-T extinction event, the Deccan Traps, a large igneous rock formation in Asia, is often blamed as the cause.
According to this theory, huge volcanoes in west-central India began to spew floods of lava during the Late Cretaceous Period.
The volcanic activity in this region could have triggered a mass extinction in different ways.
The release of dust and sulfuric aerosols from these massive volcanoes could have blocked sunlight almost completely, making it difficult for plants to carry out photosynthesis.
The collapse of the Earth’s flora would have killed off most of the animals dependent on it within a few years.
The eruptions would have also resulted in significant emissions of carbon dioxide.
Carbon dioxide acts as a heat trap (greenhouse effect), raising global temperature beyond normal levels.
The increased emissions may have also caused acid rains and raised the level of acidity in the Earth’s aquatic ecosystem.
Initially, scientists believed the Deccan Traps eruptions took place over a period of at least two million years, starting about 68 million years ago.
More recent discoveries now suggest that it spanned 800,000 years, close to the K–Pg boundary.
These new discoveries have led scientists to accept the idea that the eruption of the Deccan Traps may have been linked to an extraterrestrial impact theory.
It makes sense to assume that the impact contributed to the severity of volcanic eruptions.
Given the massive size of the Chicxulub impact, it could have triggered some of the largest eruptions Earth has ever seen, not just at the Deccan Traps but also in other parts of the planet.
This would have further exacerbated the effects of the initial impact and prolonged the planet’s recovery for several thousand years.
Climate Change and Global Cooling
Experts agree that a global climatic change occurred at the end of the Mesozoic Era.
During this time, the Earth’s environment was changing from warm and mild to a cooler one, unconducive for the dinosaurs.
The cause of this climate change and the speed of its occurrence is still being debated by scientists.
Earth’s climate has been known to change in cycles throughout geologic history.
Even the Triassic extinction event that led to the takeover of dinosaurs may have been caused by climate change.
Therefore, it is possible that climate change caused the extinction of the dinosaurs without any of the other extrinsic factors being proposed.
Alternatively, some scientists have proposed that climate change was already killing off the dinosaurs a few million years before the asteroid impact occurred.
In one study that tracked the biodiversity of the six main families of dinosaurs ten million years prior to the Chicxulub event, scientists noticed a decline in the biodiversity of dinosaurs all over the world leading up to this event.
This decline first affected the herbivores, then the population and diversity of carnivores began to decline as well.
This sort of cascading effect is consistent with the gradual collapse of ecosystems that may have been caused by drastic changes in climatic conditions.
Interestingly, only one family of dinosaurs did not show a significant decline.
These were the troodontids, generally considered the ancestors of modern birds.
The avian dinosaurs were the only group to have survived the end-Cretaceous extinction event.
The dinosaurs were mesothermic animals.
This means they need a warm climate to maintain their body temperature and keep their metabolism up.
As the temperature plunged to about seven to eight degrees Celsius, it would have severely impacted their metabolism.
Global cooling could have led to changes in vegetation patterns, affecting food availability for herbivorous dinosaurs.
As plants became less abundant or less nutritious, herbivore populations might have declined.
Climatic shifts of this nature also tend to cause habitat fragmentation.
Unable to migrate to certain regions due to unfavorable climatic conditions, dinosaur populations would have become isolated.
This could have led to lower genetic diversity, increasing vulnerability to diseases and other environmental conditions.
As resources became more scarce, habitat disruption led to increased competition among the various dinosaur species for limited food and other resources.
If the asteroid impact did happen 66 million years ago, as proposed, it met the dinosaur world completely on the verge of collapse—meaning it was simply the final straw that sent the entire Mesozoic ecosystem toppling over the edge into oblivion.
Other Theories and Hypotheses
In addition to the widely discussed impact, volcanic eruption, and climate change theories, several other lesser-known hypotheses seek to explain why the dinosaurs disappeared when they did.
While these theories have not gained as much traction as the main theories, they offer alternative perspectives on the K-Pg extinction event. Some of these include:
Methane Gas Release
This theory suggests that massive releases of methane gas from the seafloor due to tectonic activity or other causes could have contributed to the extinction of the dinosaurs.
Methane is a potent greenhouse gas, and its sudden release could have caused rapid and extreme climate changes.
But there’s limited direct evidence to support the idea of a massive methane release coinciding with the K-Pg boundary.
Additionally, it’s uncertain if such a release could have occurred on a scale large enough to cause the widespread and relatively sudden extinctions that occurred at the end of the Cretaceous Period.
Disease and Epidemics
Another commonly touted theory is that disease or epidemics was responsible for the decline of the dinosaurs.
Although diseases probably didn’t wipe out all of them, it made them more susceptible to other environmental stressors, such as changes in food availability or climate change.
Diseases could potentially explain why certain populations of herbivorous dinosaurs declined rapidly toward the end of the Cretaceous.
However, this theory is highly speculative.
There is limited direct evidence of widespread diseases causing mass extinctions during the K-Pg event or at any other time in geologic history.
It’s unlikely that a disease outbreak could have caused such a rapid and geographically widespread extinction event.
Solar Flares or Cosmic Radiation
Some theories put a spin on Alvarez’s extraterrestrial hypothesis.
Instead of an asteroid, some experts have proposed that the blast that killed off the dinosaurs was not caused by a solid projectile from space.
Instead, solar flares or cosmic radiation from space could have triggered mutations and other problems for some of the Earth’s organisms, including dinosaurs.
Space weather can potentially affect the Earth’s biosphere, which makes this a probable alternative mechanism to explain some genetic mutations.
But it’s difficult to demonstrate a direct link between these space events and worldwide extinction events, the scale of which took out the dinosaurs and several other plant and animal species.
In addition to these more probable theories, there are several more, some of which are outrightly ridiculous or even border on conspiracy theories.
The Aftermath of Extinction
Despite all the extinction events that have taken place throughout geologic history, the Earth’s ecosystem always finds a way to bounce back.
Whatever caused the extinction of the dinosaurs also led to the disappearance of up to 75% of the planet’s plants and animals.
Earth eventually recovered, but things were never the same again.
As it’s often the case after any major extinction event, the planet reorganized itself, and new organisms evolved to fill the niche left behind by the mammals.
The devastating K-T extinction event paved the way for new evolutionary opportunities and shaped the future of life on Earth in significant ways.
The immediate aftermath of the extinction event was a period of widespread ecological chaos.
Many ecological niches were left vacant due to the extinction of dinosaurs.
The “nuclear winter,” which is accounted for in every scenario of the extinction event described above, caused some sort of global cooling, disrupted food chains, and reduced sunlight.
This, in turn, severely affected photosynthesis and ecosystems in the years that followed the extinction event.
Mammals were arguably the biggest beneficiary of the extinction of the dinosaurs.
They had lived alongside the dinosaurs for millions of years but remained relatively small and inconspicuous.
This relegated them to the very bottom of the Mesozoic food chain.
With the extinction of large dinosaurs and other competitors, the mammals that survived gradually began diversifying into new forms.
These small, agile creatures began to fill roles once occupied by dinosaurs, which led to the development of mammals of varying body sizes, feeding strategies, and various forms of locomotion.
The extinction event also impacted plant life.
With the loss of large herbivores like sauropod dinosaurs, plants faced less grazing pressure as they recovered and diversified during the early Cenozoic Era.
This, in turn, created new ecosystems and provided more diverse food sources for emerging herbivorous mammals.
But the replacement of dinosaurs by mammals as the dominant group isn’t the only significant change that happened after the K-Pg extinction.
Several other groups of mammals diversified during this period too.
For instance, other reptilian groups, such as lizards and snakes, survived the extinction.
Over time they evolved into modern forms such as the monitor lizards, iguanas, and boas that still live today.
The giant boid and madtsoiid snakes were the most dominant animals for a short period after the dinosaurs disappeared.
The crocodiles were present as well, and many of them filled the vacant carnivorous niche left vacant by the dinosaurs.
In the world’s oceans, the disappearance of marine predators allowed the explosive diversification of various fish species.
In the period that followed, new groups such as billfish, eels, tunas, and flatfish emerged.
Insects have been around before the days of the dinosaurs, and many of them survived the Cretaceous extinction.
Ants became dominant and diverse during the Eocene, butterflies diversified, and mound-building termites rose to prominence in the insect world.
All was not lost for the dinosaurs.
Evidence from molecular sequencing suggests that birds are the closest things to dinosaurs we have around today.
This group of avian dinosaurs evolved as early as the Jurassic Period and survived the extinction event that killed off their terrestrial relatives.
They radiated into various forms after the K–Pg event.
They even evolved into massive predatory forms such as the terror birds (Phorusrhacidae) that were the apex predators for most of the Cenozoic Era until about 2.5 million years ago.
Ongoing Research and Discoveries
With the proposal of the impact theory in the 1980s, it seemed scientists now had a fitting answer to many of the questions relating to the extinction of the dinosaur.
The initial pushback due to the absence of an impact crater to confirm Alvarez’s theory was resolved in the decades that followed with the discovery of the Chicxulub crater.
Researchers have explored this further by conducting more detailed investigations of the Chicxulub impact site.
Advanced techniques like seismic imaging and drilling have been carried out to better understand the crater’s structure, formation, and the effects of its impact on the immediate vicinity.
These efforts aim to refine our understanding of the impact’s scale and environmental consequences.
An international panel of 41 scientists convened in 2010 to review scientific evidence related to the asteroid hypothesis.
The result of this review confirmed the asteroid hypothesis and endorsed the Chicxulub impact as the cause of the K-T mass extinction event.
In 2016, a scientific expedition drilled into the rocks of the crater impact site to obtain core samples.
The findings of that expedition are consistent with initial findings that the Chicxulub asteroid impact was a catastrophic event, big enough to have caused the extinction of the dinosaurs.
But this does not entirely rule out other theories.
Scientists are still actively studying the Deccan Traps’ volcanic activity and its potential role in the extinction event.
Efforts are ongoing to date the eruptions in this region and to further understand the impact of volcanic gasses on global climate.
Researchers are working to reconstruct past climate conditions and explore connections between volcanic activity and rapid climate change.
Beyond the causes of the extinction event, paleontologists continue to discover new fossil sites and analyze existing ones to better understand how different species were affected by the extinction event.
Particularly instrumental to this are microfossils, ancient pollen, and sediments that can provide insights into changes in ecosystems and biodiversity as the planet transitioned from the Cretaceous to the Paleogene period.
The K-T extinction event marked the end of one of the planet’s most dominant animals since terrestrial life began.
Different hypotheses have been proposed to explain the sudden disappearance of the non-avian dinosaurs from the fossil record about 66 million years ago.
Everyone agrees that a major climatic change took place around this time that may have caused the sudden disruption of the Earth’s ecosystem.
But the exact cause of this decline is still subject to scientific debate.
The most prevalent theory is that a single catastrophic asteroid impact event led to the extinction of the dinosaurs.
The Chicxulub asteroid impact threw dust, ash, and toxic gasses into the air that blocked sunlight.
It also triggered wildfires, tsunamis, and other catastrophes that killed off dinosaurs in their numbers.
An alternative theory blames volcanic activities, specifically relating to the formation of the Deccan Traps, for the event.
The 66-million-year-old igneous formation is one of the largest volcanic features on Earth, and its formation may have thrown Earth’s climate off course for several years afterward.
But these theories don’t have to be exclusive to each other.
The impact, volcanic activity, and climate change theory can all be correct.
All of these events likely interacted to create a complex web of stressors that contributed to the mass extinction of the dinosaurs.
By integrating all of these factors, the dinosaurs simply didn’t stand a chance against mother nature’s deadly blow.